1. Field of the Invention
The present invention relates to an electrophotographic printer composed of a photoconductor, a developing device, and a cleaning device and particularly to an electrophotographic printer which can reuse developer remaining on a photosensitive drum.
2. Technology Review
Electrophotographic printers employ a method of printing comprising the steps of exposing a charged photoconductor to an exposing light source to form an electrostatic latent image on the surface of the photoconductor, developing the latent image by applying toner powder thereto, and thereafter transferring the thus developed toner image onto a recording medium such as a recording paper.
FIG. 9 is a schematic side view of an exemplified arrangement of such an electrophotographic printer. In the figure, denoted at 1 is a photosensitive drum having on the surface thereof a photosensitive semiconductor such as a Se photoconductor, a Cds photoconductor, an amorphous Silicon photoconductor, an organic photoconductor or (OPC); 2 is a first electrifier; and 3 is light for forming an image coming from a light source, not shown. Denoted at 4 is a developing device comprising a toner tank 4a disposed therein for storing toner, 5 is a sheet guide, 6 is a transferring electrifier, 7 is a fixing device, and 8 is a light source for removing electrical charge. Denoted at 9 is a cleaning device, 10 is a developing roller disposed in the developing device 4, 11 is a recording paper, and 12 is a feed roller for feeding the recording paper 11.
In the thus constructed electrophotographic printer, the photosensitive drum 1 is rotated at a given speed in the direction of the arrow R so that the surface of the photosensitive drum 1 is uniformly electrified by the first electrifier 2. A latent image is formed on the surface of the electrified photosensitive drum 1 by radiating light 3 from an exposing light source. The thus formed latent image reaches a position opposing the developing roller 10 in the developing device 4 as the photosensitive drum 1 is rotated in the direction of the arrow R. The toner supplied from a toner reservoir by way of the developing roller 10 adheres to the photosensitive drum 1 to make the visible latent image.
Recording paper 11 which is fed sheet by sheet from a paper feed portion by way of the feed roller 12 are transported to the upper portion of the transferring electrifier 6 along a sheet guide 5 in accordance with the rotation of the photosensitive drum 1. The toner image formed on the photosensitive drum 1 is transferred onto the recording paper 11 by the transferring electrifier 6. The recording paper 11 on which the toner image is transferred but not fixed is transported toward a fixing device 7 by a transporting means, not shown, and the toner image is fixed on the recording paper 11 by being heated or pressed in the fixing device 7 to complete printing.
The latent toner image on the photosensitive drum 1 is thereafter erased by irradiation from the light source 8. The toner which remains on the photosensitive drum 1, instead of being transferred onto the recording paper, is removed and collected by the cleaning device 9. The photosensitive drum 1 is electrified again by the first electrifier 2 for successive image forming processing after complete removal of the toner by the cleaning device 9.
The toner collected in a collecting case 9a provided in the cleaning device 9 is regularly removed by an operator or a person in charge of maintenance of the electrophotographic printer.
However, when the electrophotographic printer is used frequently or for a long time, the increased quantity of collected toner causes a frequent, troublesome dumping operation. As a result, a large collecting case has to be used in order to reduce the frequency of the dumping operation, thus resulting in a large sized electrophotographic printer.
Consequently, an electrophotographic printer in which the toner collected by the cleaning device 9 is recycled to the developing device 4 has been provided recently.
FIG. 10 is a schematic side view showing an arrangement of such an exemplified electrophotographic printer in which the toner is recycled to the developing device. As illustrated in the figure, the remaining toner 19 which has not been transferred onto the recording paper is transported to the cleaning device 20 while the electric charge on the photosensitive drum 1 is discharged by a light source 8, with the rotation of the photosensitive drum 1 in the direction of the arrow R. For collecting the toner 19, there are various methods such as using a blade or a brush. An embodiment using a brush will now be described.
A cleaning brush denoted at 21 scrapes off the remaining toner 19 from the photosensitive drum 1 as it rotates in the direction of the arrow A, to transfer the toner 19 onto a collecting roller 22. At this time, a blade 23 scrapes off the toner 19 causing it to fall into a guide 24 after the toner 19 is transferred from the cleaning brush 21 to the collecting roller 22 by an electrostatic effect (force).
An Auger shaft, feed spring, etc. (not shown), provided on the lower portion of the guide 24 transport the dropped toner 19 toward a sending portion 25a. The toner 19 is transported from the sending portion 25a into a guide pipe 25. Another Auger shaft (not shown) is provided to transport the toner 19 by the rotation thereof to a conveying in portion 25b provided on the developing device 4. The toner 19 is recycled into the developing device 4 from the conveying in portion 25b. As described above, the toner 19 collected by the cleaning device 20 is recycled to the developing device 4 by way of the guide pipe 25 for reuse in developing. The recycling of the toner 19 makes it unnecessary to carry out the troublesome dumping operation and can reduce the amount of toner consumed in the electrophotographic printer.
There are, however, the following three problems in the conventional electrophotographic printer described above in which the remaining toner is recycled to the developing device 4 for reuse.
The first problem is that the electrophotographic printer has a wide width and needs a large floor area for its installation since the guide pipe 25 for transporting the collected toner from the cleaning device 20 to the developing device 4 is provided aside the photosensitive drum 1.
The second problem is that the toner collected is unevenly distributed (piled locally) in the side edge portion of the developing device 4 since the toner flows back into the developing device 4 from the conveying in portion 25b in the side edge portion of the developing device 4.
The third problem is that the collected toner clogs the guide pipe 25.
These problems will be described more in detail hereinafter with reference to FIG. 11 which is a schematic plan view showing the arrangement of the electrophotographic printer illustrated in FIG. 10 in which the toner is recycled to the developing device 4 for reuse.
The first problem comes from the fact that the guide pipe 25 is provided aside the photosensitive drum 1, as easily understood from FIG. 11. Generally the width of the recording portion of an electrophotographic printer is determined by the width of the photoconductor, i.e., the photosensitive drum 1. This is because the developing roller 10 housed in the developing device 4 and the cleaning brush 21 provided in the cleaning device 20 are shorter than the photosensitive drum 1 in width. As a result, if the collected toner recycling route is designed so as to pass aside the photoconductor, the electrophotographic printer becomes wider by the width of the guide pipe 25, as understood from FIG. 11.
The second problem comes from the fact that the collected toner is carried back to the side edge portion of the developing device 4 from the conveying in portion 25b of the guide pipe 25, as understood from FIG. 11.
Toner is stored uniformly in the width direction of the developing device 4 when it is supplied thereto, and it is fed to an electrostatic latent image on the photoconductor by way of the developing roller 10.
However, the collected toner which has been carried back by way of the guide pipe 25, is carried back to the side edge portion of the toner containing portion of the developing device 4, so that the toner is piled in the vicinity of the side edge portion as described above, and is not distributed evenly in the toner containing portion. When the toner is not distributed evenly in the developing device, it is difficult to correctly detect the presence or absence of toner in the developing device. That is, the presence of remaining toner is reported when the sensor detects the presence of toner in the vicinity thereof, and the absence of toner is reported when the sensor does not detect the toner in the vicinity thereof. But if the toner is not evenly distributed (if much toner remains in the side edge portion as described above) in the developing device, it sometimes occurs that there remains very little toner on the opposite side of the sensor (the opposite side of the side edge portion 25b in the developing device 4 to which the toner is carried back) even if the sensor detects toner in the vicinity thereof, which results in the deterioration of printing quality (partial reduction in printing density).
Furthermore, inasmuch as the collected toner is electrically charged at a reverse electrical potential, its major portion is different in electric potential from that first supplied to the developing device. As a result, printing quality is liable to be partially deteriorated, which is caused by the difference in electric charge between the collected toner and the first stored toner when they are unevenly distributed in the developing device.
Also, when the toner is removed from the photoconductor by the cleaning device, powders of paper which has adhered to the photoconductor are also swept together with the toner, and are carried back to the developing device 4 by way of the guide pipe 25 with the toner. Other than the powders of paper, an antistatic agent which was primarily added to the toner and came off in the course of printing is also collected by the cleaning device and carried back to the developing device. As a result, they cause blurred printing or uneven density in printing when they are unevenly distributed in the developing device 4 although it does not matter when they constitute a very small amount and are evenly distributed therein.
The third problem is caused by the friction between the toner and the inner surface of the guide pipe 25, the electric charge due to the friction between the toner and the Auger shaft for feeding and the deformation and deterioration of the toner due to the friction between the toner and the guide pipe 25 or Auger shaft. The electrostatic charge due to friction causes condensation of the toner itself, which deteriorates its fluidity, or the adhesion of the toner to the inner surface of the guide pipe 25 by electrostatic force, which deteriorates its fluidity locally. The deformation of toner particles also deteriorates its fluidity so that toner fed one after another are caught in the portion where the fluidity of the toner is deteriorated and finally clogs the guide pipe 25. When the guide pipe 25 is clogged by toner, the flow of toner is stopped entailing the inundation of toner in the cleaning device and dirty printing caused by insufficient cleaning.